I’ve added boot.kernelModules = [ "xpad" ]; to my configuration.nix, however on boot the xpad module does not start up. If I manually modprobe it, controllers all work, but the module doesn’t auto load and I have no clue where to go from here.
Have you taken a look at your logs? Maybe xpad is just failing to start.
Can you post your journalctl and dmesg log to pastebin.com?